Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/36

24 Cobalti-cyanide of potassium, (NC) 6 Co'".K 3 , forms yellow crystals isomorphous with those of red prussiate (see infra). It is a re- markably stable salt. In its behaviour to reagents it exhibits none of the characters of a cobalt salt or of a simple cyanide. Aque- ous mineral acids convert it into the hydrogen salt (NC) 6 Co'"H 3 , which remains undecomposed on boiling. Heavy metallic salts pro- duce precipitates of cobalti-cyanides ; for example, (NC) 8 Co'".Ag 3 . (8) Ferrosum. See " prussiate of potash " above. (9) Ferricum. Ferric hydrate and ferric compounds generally do not act upon cyanide of potassium in a manner analogous to that of ferrous com- pounds ; but a ferri-cyanide analogous to the cobalti-salt referred to in (7) is readily produced by passing chlorine into a cold solution of ordinary prussiate, (NG) 6 Fe . K 4 + Cl - KC1 + (NC) 6 fe'". K 3 . 1 I n preparing the salt an excess of chlorine and elevation of tempera- ture must be avoided, or else part of the salt is decomposed with formation of a green precipitate. The solution on evaporation and cooling yields splendid dark red crystals, soluble in 2 "54 parts of water of 15 '6 C. (Wallace), forming a most intensely yellow solution. (Ordinary prussiate solution is only pale yellow even when saturated in the cold. ) This salt (discovered by L. Gmelin in 1822) is now being manufactured industrially and is known in commerce as "red prussiate." In its reactions it is analogous to ordinary yellow prussiate. The same group, (NC) 6 Fe, which in the latter acts as a four-valent, in the red salt plays the part of a tri-valent radical, (NC) 6 fe. But the radical thus modified has a great tendency to assume the four-valent form; hence an alkaline solution of red prussiate is a powerful oxidizing agent, (NC) 6 fe.K 3 + KHO = (NC) 6 Fe. K 4 + HO. The HO goes to the reduc- ing agent. Like the yellow salt, red prussiate is not poisonous, at least when pure. Ferro- aiid Ferri-cyanides of Iron. The two prussiates are con- stantly being used in the laboratory as very delicate reagents for the detection of iron salt, and for the discrimination of ferrous and ferric compounds in solutions, (1) ferro-cyanide and ferrous salt, white precipitate ; (2) ferri-cyanide and ferric salt, intensely brown coloration ; (3) ferro-cyanide and ferric salt, blue precipitate ; (4) ferri-cyanide and ferrous salt, blue precipitate. These blue precipitates are being produced industrially and used as pigments, under the names of "prussian blue" and "Turnbull's blue" for (3) and (4) respectively. The latter has been thus known for now half a century ; yet the constitution of the precipitates and the true rationale of their formation have been fully cleared up only during the last few years. The main results of the researches referred to are included in the following paragraphs. (1) Ferro-cyanide of Hydrogen, (NC) 6 Fe.H 4 , is obtained as a white crystalline precipitate when air-free concentrated solution of yellow precipitate is mixed with hydrochloric acid and ether. It is easily soluble iii water and in alcohol. An aqueous solution of it is pre- pared for technical purposes by mixing a strong solution of yellow prussiate with enough tartaric acid to bring down the potassium as cream of tartar. When the solution of this ferro-hydrocyanic acid is boiled half the cyanogen goes off as NCH, while the other remains as part of a white, rather unstable, precipitate, (NC) 6 Fe . j^ e . When the solution is exposed to the air, especially at higher temperatures, part of the cyanogen goes off as NCH, another part suffers oxidation into H.,0 + NC, and this latter combines with the Fe(NC) 2 of the original compound into blue bodies similar in their general properties to prussian blue. This latter change is utilized in calico-printing for producing patterns of, or dyeing with, prus- sian blue. The white precipitate (NC) 6 Fe.Tr may be looked upon as an acid of which -, 8 (2) Everett's Salt, (NC) 6 Fe . g, is the potash salt. This salt is produced in the ordinary process for making prussic acid (see above). It is probably identical with the white precipitate produced when ferrous salt is decomposed by prussiate of potash. Everett's salt when exposed to the air quickly absorbs oxygen and becomes blue ; the reaction, as Williamson showed, assumes a simple form when the precipitate is boiled with nitric acid. One-half of the potassium is then oxidized away, and a blue double ferri-cyanide of potassium and ferrosum takes the place of the original precipitate : (NC) 6 Fe . KoFe^KjjO as nitrate) + {(NC) 6 fe) "'Fe"K'. Williamson's blue. This blue when boiled with ferro-cyanide of potassium is reconverted into the original Everett's salt with formation of a solution of red prussiate (NC) 6 fe.KFe + K 2 *K 2 . Fe(NC) 6 = (NC) 6 fe. K 3 + Fe(NC) 6 . FeK* Red prussiate. Everett's salt. the asterisked radicals changing places. (3) Soluble Prussian Blue is isomeric with Williamson's blue. It is produced by mixing a solution of ferric salt with excess of yellow prussiate, which, however, is an old process ; what has been aseer- 1 Here we use the symbol " fe " as designating 56 parts of ferric iron," Fe " meaning the same quantity of ferrosum. tained lately is that the very same precipitate is produced by addi- tion to a ferrous salt of an excess of red prussiate. II. (NC) 6 Fe.K 4 + feCl 3 = 3KCl + (NC) 6 Fe. Kfe = B". B' and B" in the formulae look different, but the difference is only apparent ; in either case the group (NC) 6 is combined with IFe and Ife and IK ; the bodies are identical (Skraup ; Reindel). The precipitate B, though insoluble in salt solutions, is soluble in pure water, forming an intensely blue solution ; hence the name. Now the potassium in soluble prussian blue can be displaced by iron in two ways, namely, by digestion with solutions of ferrous or ferric salts. In the former case (NC) 6 feFeK becomes (NC) 6 feFen, or empirically (NC) 12 Fe 5 ; this is Gmelin's ("Turnbull's") blue. In the latter case (NC) 6 FefeK becomes (NC) 6 Fefe, or empirically (NC) 18 Fe- ; this is prussian blue as discovered by Diesbach. Contrasting this latter formula with that of Gmelin's blue (NC) 18 Fe 7 j, we see that the latter needs only lose ^Fe to become prussian blue ; this sur- plus iron in fact can be withdrawn by means of nitric acid. In the manufacture of prussian blue the general process is to first precipitate ferrous sulphate with yellow prussiate and then to fully oxidize the precipitate by means of nitric acid or chlorine as far as the oxygen of the air does not do it. The following receipt is recommended amongst others. Six parts each of green vitriol and yellow prussiate are dissolved separately, each in fifteen parts of water, and the solutions mixed. One part of concentrated sul- phuric acid and twenty-four parts of fuming muriatic acid are then added, and after standing some hours also a solution of bleaching powder in instalments until the blue colour is fully developed. " Turnbull's " blue is made by precipitating red prussiate of potash with excess of ferrous salt ; but it is easily seen from what was said above that the use of this relatively expensive double cyanide might be dispensed with. The properties of the two pigments are pretty much the same. They are sold in the form of solid cakes or lumps, which, in addition to their blue colour, present a coppery lustre on fracture. They are stable against acids, but sensibly affected (bleached) on prolonged exposure to sunlight ; and, although they stand neutral soap fairly well, they are decomposed promptly by solutions of even the carbonates of the alkalis with formation of hydrated oxides of iron. The cheaper commercial varieties are more or less largely diluted with clay, sulphate of baryta, &c. Pure prussian blue dissolves readily in a dilute solution of oxalic acid ; the intensely blue solution used to serve as a blue ink, but has come to be superseded by the several more brilliant blues of the coal-tar series. These tar-blues have displaced prussian blue also in other applications, and as a commercial pigment it has besides to straggle against ultramarine. In short, it has gone very much out of use, and as a consequence the manufacture of yellow prussiate is no longer so remunerative as it used to be. Analysis of Cyanides. As hydrocyanic acid and cyanide of potassium are dangerously poisonous, and the latter at least is easily procured in commerce, the detection of cyanogen in this state of combination is one of the problems of forensic chemistry. To detect such cyanogen in, say, the contents of a stomach the first step is to distil the mass after acidification with tartaric acid, which decomposes cyanide of potassium but does not liberate prussic acid from prussian blue (or even prussiate of potash ?). If the dis- tillate gives no precipitate with nitrate of silver hydrocyanic acid is absent, if it does the precipitate may have been produced by hydrochloric acid, which may then be eliminated by redistillation with borax or sulphate of soda, neither of which affects NCH. But even in the presence of chlorides the following two tests give perfect certainty. (1) A solution of hydrocyanic acid, when Blkalinized with caustic potash and then mixed with, first ferroso-ferric salt and then excess of hydrochloric acid, gives a precipitate, or at least a green suspension, of prussiau blue. (2) A solution of NCH, when mixed with ammonia and yellow sulphide of ammonium, is changed into one of sulphocyanate of ammonium, which, after removal of the excess of reagents by evaporation at a gentle heat, strikes an intense and very characteristic red colour with ferric salts, which colour does not vanish (as that of ferric acetate does) on even strong acidification with mineral acid (Liebig's test). The quantitative determination of cyanogen given as an aqueous solution of hydrocyanic acid or cyanide of potassium can (if haloids are absent) be effected by adding excess of nitrate of silver, then acidifying, if necessary, with nitric acid, filtering off, washing, drying, and weighing the cyanide of silver produced. AgNC= 134 corresponds to NCH = 27 parts. A more ex- peditious method lias been invented by Liebig. A known quantity of the given prussic acid is alkalinized strongly with caustic potash and then diluted freely with water. The caustic alkali usually contains plenty of chloride as an impurity, else a little alkaline chloride must be added. A standard solution of nitrate of silver (conveniently adjusted so as to contain 6 '30 grammes of fused ni- trate per 1000 cubic centimetres, equivalent to 2 grammes of NCH) is now dropped in from a burette until the cloud of chloride of silver which appears locally from the first just fails to disappear on stirring, i.e., until the reaction 2KNC -t- AgN0 3 = KAg.(NC) 2 + KN0 3